Since the introduction of the first implantable pacemakers in the 1960's, there have been considerable advancements both in the field of electronics and the field of medicine, such that there is presently a wide assortment of commercially-available implantable medical devices. The class of implantable medical devices now includes not only pacemakers, but also implantable cardioverters, defibrillators, neural stimulators, and drug administering devices. Today's state-of-the-art implantable medical devices are vastly more sophisticated and complex than early pacemakers, capable of performing significantly more complex tasks. The therapeutic benefits of such devices have been well-proven.
As the functional sophistication and complexity of implantable medical devices has increased over the years, it has become increasingly more important for such devices to be equipped with a telemetry system for enabling them to communicate with an external unit.
For example, shortly after the introduction of the earliest fixed-rate, non-inhibited pacemakers, it became apparent that it would be desirable for a physician to non-invasively exercise at least some amount of control over the device, e.g., to turn the device on or off or adjust the fixed pacing rate, after implant. In early devices, one way the physician was able to have some control over implantable device operation was through the provision of a magnetic reed switch in the implantable device. After implant, the reed switch would be actuated by placing a magnet over the implant site. Reed switch closure could then be used, for example, to alternately activate or deactivate the device. Alternatively, the fixed pacing rate of the device could be adjusted up or down by incremental amounts based upon the duration of reed switch closure. Many different schemes utilizing a reed switch to adjust parameters of implanted medical devices have been developed. See, for example, U.S. Pat. No. 3,311,111 to Bowers, U.S. Pat. No. 3,518,997 to Sessions, U.S. Pat. No. 3,623,486 to Berkovits, U.S. Pat. No. 3,631,860 to Lopin, U.S. Pat. No. 3,738,369 to Adams et al., U.S. Pat. No. 3,805,796 to Terry, Jr., and U.S. Pat. No. 4,066,086 to Alferness et al.
As new, more advanced features are incorporated into implantable devices, it is typically necessary to convey correspondingly more information to the device relating to the selection and control of those features. For example, if a pacemaker is selectively operable in various pacing modes (e.g., VVI, VDD, DDD, etc . . . ), it is desirable that the physician or clinician be able to non-invasively select a mode of operation. Similarly, if the pacemaker is capable of pacing at various rates, or of delivering stimulating pulses of varying energy levels, it is desirable that the physician or clinician be able to select, on a patient-by-patient basis, appropriate values for such variable operational parameters.
Even greater demands are placed upon the telemetry system in implantable devices having such advanced features as rate adaptation based upon activity sensing, as disclosed, for example, in U.S. Pat. No. 5,052,388 to Sivula et al. entitled "Method and Apparatus for Implementing Activity Sensing in a Pulse Generator", in U.S. Pat. No. 5,266,413 issued to Bennett et al, entitled "Rate Responsive Pacemaker and Method for Automatically Initializing the Same", and in U.S. patent application Ser. No. 07/880,877, filed May 11, 1992 in the name of Shelton et al., entitled "Work-Modulated Pacing Rate Deceleration". The Sivula et al. '388 patent, the Bennett et al '413 patent and the Shelton et al. '877 application are each hereby incorporated by reference herein in their entireties.
The information which is typically communicated to the implantable device in today's state-of-the-art pacemakers includes: pacing mode, multiple rate response settings, electrode polarity, maximum and minimum pacing rates, output energy (output pulse width and/or output current), sense amplifier sensitivity, refractory periods, calibration information, rate response attack (acceleration) and decay (deceleration), onset detection criteria, and perhaps many other parameter settings.
The need to be able to communicate more and more information to implanted devices quickly rendered the simple reed-switch closure arrangement insufficient. Also, it has become apparent that it would also be desirable not only to allow information to be communicated to the implanted device, but also to enable the implanted device to communicate information to the outside world.
For diagnostic purposes, for example, it is desirable for the implanted device to be able to communicate information regarding its operational status to the physician or clinician. State of the art implantable devices are available which can even transmit a digitized ECG signal for display, storage, and/or analysis by an external device.
As used herein, the terms "uplink" and "uplink telemetry" will be used to denote the communications channel for conveying information from the implanted device to an external unit of some sort. Conversely, the terms "downlink" and "downlink telemetry" will be used to denote the communications channel for conveying information from an external unit to the implanted device.
Various telemetry systems for providing the necessary communications channels between an external unit and an implanted device have been shown in the art. Telemetry systems are disclosed, for example, in the following U.S Pats: U.S. Pat. No. 4,539,992 to Calfee et al. entitled "Method and Apparatus for Communicating With Implanted Body Function Stimulator"; U.S. Pat. No. 4,550,732 to Batty, Jr. et al. entitled "System and Process for Enabling a Predefined Function Within An Implanted Device"; U.S. Pat. No. 4,571.589 to Slocum et al. entitled "Biomedical Implant With High Speed, Low Power Two-Way Telemetry"; U.S. Pat. No. 4,676,248 to Berntson entitled "Circuit for Controlling a Receiver in an Implanted Device"; U.S. Pat. No. 5,127,404 to Wyborny et al. entitled "Telemetry Format for Implanted Medical Device"; U.S. Pat. No. 4,211,235 to Keller, Jr. et al. entitled "Programmer for Implanted Device"; U.S. Pat. No. 4,374,382 to Markowitz entitled "Marker Channel Telemetry System for a Medical Device"; and U.S. Pat. No. 4,556,063 to Thompson et al. entitled "Telemetry System for a Medical Device".
Typically, telemetry systems such as those described in the above-referenced patents are employed in conjunction with an external programming/processing unit. One programmer for non-invasively programming a cardiac pacemaker is described in its various aspects in the following U.S. Patents to Hartlaub et al., each commonly assigned to the assignee of the present invention and each incorporated by reference herein: U.S. Pat. No. 4,250,884 entitled "Apparatus For and Method Of Programming the Minimum Energy Threshold for Pacing Pulses to be Applied to a Patient's Heart"; U.S. Pat. No. 4,273,132 entitled "Digital Cardiac Pacemaker with Threshold Margin Check"; U.S. Pat. No. 4,273,133 entitled Programmable Digital Cardiac Pacemaker with Means to Override Effects of Reed Switch Closure"; U.S. Pat. No. 4,233,985 entitled "Multi-Mode Programmable Digital Cardiac Pacemaker"; U.S. Pat. No. 4,253,466 entitled "Temporary and Permanent Programmable Digital Cardiac Pacemaker"; and U.S. Pat. No. 4,401,120 entitled "Digital Cardiac Pacemaker with Program Acceptance Indicator".
Aspects of the programmer that is the subject of the foregoing Hartlaub et al. patents (hereinafter "the Hartlaub programmer") are also described in U.S. Pat. No. 4,208,008 to Smith, entitled "Pacing Generator Programming Apparatus Including Error Detection Means" and in U.S. Pat. No. 4,236,524 to Powell et al., entitled "Program Testing Apparatus". The Smith '008 and Powell et al. '524 patents are also incorporated by reference herein in their entirety.
Most commonly, telemetry systems for implantable medical devices employ a radio-frequency (RF) transmitter and receiver in the device, and a corresponding RF transmitter and receiver in the external programming unit. Within the implantable device, the transmitter and receiver utilize a wire coil as an antenna for receiving downlink telemetry signals and for radiating RF signals for uplink telemetry. The system is modelled as an air-core coupled transformer. Examples of such a telemetry system are shown in the above-referenced Thompson et al. '063 and Hartlaub et al. '120 patents.
In order to communicate digital data using RF telemetry, a digital encoding scheme such as is described in U.S. Pat. No. 5,127,404 to Wyborny et al. entitled "Improved Telemetry Format" is used. In particular, for downlink telemetry a pulse interval modulation scheme may be employed, wherein the external programmer transmits a series of short RF "bursts" or pulses in which the during of an internal between successive pulses (e.g., the interval from the trailing edge of one pulse to the trailing edge of the next) encodes the data. In particular, a shorter interval encodes a digital "0" bit while a longer interval encodes a digital "1" bit.
For uplink telemetry, a pulse position modulation scheme may be employed to encode uplink telemetry data. For pulse position modulation, a plurality of time slots are defined in a data frame, and the presence or absence of pulses transmitted during each time slot encodes the data. For example, a sixteen position data frame may be defined, wherein a pulse in one of the time slots represents a unique four bit portion of data. The Wyborny et al. '404 patent is hereby incorporated by reference herein in its entirety.
Programming units such as the above-described Hartlaub et al. Programmer typically interface with the implanted device through the use of a programming head or programming paddle, a handheld unit adapted to be placed on the patient's body over the implant site of the patient's implanted device. A magnet in the programming head effects reed switch closure in the implanted device to initiate a telemetry session. Thereafter, uplink and downlink communication takes place between the implanted device's transmitter and receiver and a receiver and transmitter disposed within the programming head.
For programming arrangements such as the one just described, both uplink and downlink telemetry signal strength vary as a function of programming head positioning. Thus, it is important for the programming head to be properly positioned over the patient's implant site, not only so that the magnet in the programming head is close enough to the implanted device to cause reed switch closure, but also so that the downlink RF signals can be detected in the implanted device and the uplink signals can be detected by the programming head. If the programming head is too far away from the implanted device, the attenuation of RF signals transmitted across the boundary of the patient's skin may be too great, preventing the telemetry link from being established.
Although both uplink and downlink signal strength vary as a function of head position, the coupling maps for uplink and downlink telemetry may be different. That is, what may be optimal positioning for uplink telemetry may be less optimal for downlink telemetry, and vice versa.
Differences between the uplink and downlink telemetry coupling maps arise when a so-called "dual-coil" system is employed in the programmer. An example of a dual-coil telemetry system is described in U.S. Pat. No. 4,542,532 to McQuilkin, entitled "Dual Antenna Receiver". The McQuilkin '532 patent is hereby incorporated by reference herein in its entirety. In a dual-coil system, two coils are connected in series opposition to achieve noise cancellation in the receive mode. The two coil series-opposing configuration makes the programmer sensitive to the curl of magnetic fields. Such curl sensitivity results in a significant increase in noise rejection over a single-coil antenna for in-band, spatially-aligned interference fields present in the proximity of the antenna.
For transmission from a dual-coil antenna, the two coils are configured in "parallel aiding" fashion, such that the magnetic field transmitted by the antenna is effectively doubled.
Often, medical device programmers, for example the Model 9710 or 9760 programmers commercially available from Medtronic, Inc., are provided with a Head Positioning Indicator, either audible or visible, for indicating to the physician or clinician when the programming head is properly located over a patient's implanted device. In the prior art, the technique most commonly used for determining when the programming head is properly positioned can be characterized generally as "open loop", in that the determination of correct head positioning is based solely upon an assessment of whether the uplink signal (i.e., the signal transmitted from the implanted device to the external programming head) meets some minimum requirement. In particular, uplink signal strength is measured based upon the operating gain of the uplink telemetry receiver. In such open loop verification systems, adequate downlink signal strength is not tested.
An example of an open loop system for determining the proper positioning of a programming head is described in U.S. Pat. No. 4,531,523 to Anderson, entitled "Digital Gain Control for the Reception of Telemetry Signals From Implanted Medical Devices". The '523 patent relates to a system wherein verification of the programming of an implanted device is provided by the transmission of predetermined signals from the implanted device. When errors are detected in these uplink signals, the number of errors are counted over a period of time. If more than a predetermined number of errors occur in that time, the gain of the programming unit receiver is adjusted downward by a predetermined amount. This adjustment continues until uplink signals are received without error. As an open loop system, however, the system of the '523 patent does not test for errors in downlink telemetry signals, and does not evaluate the strength of the received downlink telemetry signals.
When downlink signal strength cannot be tested, it is important for the physician or clinician to be able to otherwise verify that programming signals transmitted from the programming head are accurately received and processed by the implanted device. To this end, a system is described in the above-referenced Hartlaub et al. '120 patent wherein circuitry in the implanted device performs several different checks on the detected downlink programming signal, including a parity check and an access code check, and issues a program acceptance signal if the downlink programming is found to be valid.
As those of ordinary skill in the art will appreciate, a communications protocol using handshaking can verify that a minimum downlink field strength for detection in the implanted device exists prior to signalling the physician or clinician that correct head positioning has been achieved. However, a handshaking protocol cannot provide any information useful for optimization of head positioning to ensure an adequate operating margin. This means that proper programming head positioning may be indicated even though the programming head is actually marginally positioned, such that a very slight shift in positioning (e.g., due to patient motion) results in downlink telemetry failure.
Those of ordinary skill in the art will appreciate that one possible way to ensure an adequate margin between the strength of detected downlink signals in an implanted medical device and the device's detection threshold would be to transmit downlink telemetry signals having much larger than nominal amplitudes. From an energy consumption standpoint, this solution is particularly feasible when used in the context of a line-powered (as opposed to battery-powered) external programming unit, since for a line-powered programming unit, energy consumption is not a critical factor. If extremely strong downlink signals were transmitted, the programmer could be assured that the signals will be strong enough to be detected by the implanted device. In this way, the need for a downlink signal strength indication would be obviated.
There are several disadvantages, however, with the use of excessively strong downlink telemetry signals to ensure their detection by the implanted device. First, while power consumption is not a crucial factor in line-powered programmers, it is becoming increasingly common for programming units to be portable and battery-powered, so that they may be easily transported and used in a variety of clinical and/or non-clinical settings. For battery-powered programmers, it would be inefficient and undesirable to consume the limited battery power with unnecessarily high-level downlink signals.
Perhaps a more critical disadvantage of transmitting high-level downlink signals is the possibility that the large RF energy bursts in the downlink transmission may interfere with the operation of the implanted device. In particular, for very high-energy downlink telemetry pulses, it is possible for the downlink signal to induce voltages on implanted pace/sense leads. Such induced voltages may be interpreted by the implanted device's sensitive sensing circuitry (e.g., pacemaker) as cardiac events and may thereby cause pacemaker inhibition or lead to loss of synchronization with intrinsic cardiac activity. This problem is likely to worsen as improved (i.e., more sensitive) sensing circuitry is developed. It is also possible that when downlink telemetry is performed continuously for long periods of time (e.g., during clinical diagnostic tests on the patient), that excessively large downlink signals could cause thermal complications, i.e., cause the implanted device to exhibit an undesirable increase in temperature.
In the prior art, implanted devices have been provided with clamping diodes to prevent overdriving of the implanted devices' telemetry inputs and circuitry to dissipate energy induced in the implanted downlink telemetry receiver coil. For example, energy induced in a device's receiver coil can to a limited extent be redirected to the device's battery.
Nonetheless, problems with excessive energy from downlink telemetry are likely to be exacerbated in state-of-the-art and future devices to which more and more information must be communicated over relatively long periods of time.
A further problem arising from the inability, in prior art programming arrangements to ascertain the strength of the downlink signal as detected by the implanted device is that telemetry failures are difficult to troubleshoot. This is due, in part, to the fact that when downlink signals are not successfully received, the programmer cannot tell whether the problem lies in the positioning of the programming head, in inadequate downlink signal strength, or elsewhere.
The use of a secondary (feedback) sensing coil within the programmer itself to sample the downlink signal intensity within the programming head compensates for such variables as supply voltage variation, temperature-induced variation, parts tolerance variation, transmit antenna detuning and transmit antenna loading. However, a sensing coil does not compensate for field distortions beyond the programming head that do not result in intensity changes in the feedback sensing coil. The field is merely standardized and does not dynamically adjust to compensate for field distortion or attenuation that occurs in and around the implanted device itself, nor for the alignment of the electromagnetic field vector with the implanted device's downlink sensing vector.
As a result of the foregoing considerations, it is believed that it would be desirable to provide a programmer with the capability of evaluating the strength of downlink telemetry signals as detected by the implanted device, i.e., to enable the implanted device to communicate to the programmer information about the strength of downlink signals after they have crossed the implant boundary. Such capability would be particularly desirable in the context of dual-coil systems, wherein the uplink and downlink coupling maps are different due to the different coil configurations used in the programmer for transmission and reception. With such a capability, the programmer could dynamically adjust the amplitude of downlink telemetry pulses, such that downlink signals would be transmitted at a level known to exceed the implanted device's detection threshold. At the same time, the programmer would minimize transmission of excessively or unnecessarily large downlink signals which could lead to the aforementioned problems with telemetry receiver overdriving, pacemaker inhibition due to induced voltages applied to the sensing circuitry, and the like.
If the received downlink can be controlled, programmers with very high output level capabilities could be used to extend the range of the system.